Synchronization system and method for achieving low power battery operation of a vehicle locating unit in a stolen vehicle recovery system which receives periodic transmissions

ABSTRACT

Synchronization for achieving low power battery operation of a vehicle locating unit in a stolen vehicle recovery system whose radio receiver receives periodic transmissions, includes receiving periodic transmissions; turning on a radio receiver for a limited time to detect an expected message; if an expected message is not found, turning off the receiver and turning it on again after a time asynchronous with the transmission period; and after finding an expected message, waiting for the period of the transmissions less the length of an expected message and then looking for a synchronization symbol in the expected message and synchronizing subsequent actuation of the receiver using that synchronization symbol.

RELATED APPLICATIONS

This application claims benefit of and priority to U.S. ProvisionalApplication Ser. No. 61/279,630 filed Oct. 23, 2009 under 35 U.S.C.§§119, 120, 363, 365, and 37 C.F.R. §1.55 and §1.78 incorporated hereinby this reference.

FIELD OF THE INVENTION

This invention relates to a synchronization system for achieving lowpower battery operation of a vehicle locating unit in a stolen vehiclerecovery system which receives periodic transmissions.

BACKGROUND OF THE INVENTION

The applicant's successful and popular vehicle recovery system soldunder the trademark LoJack® includes a small electronic vehicle locatingunit (VLU) with a transponder hidden within a vehicle, a private networkof communication towers each with a remote transmitting unit (RTU), oneor more law enforcement vehicles equipped with a vehicle tracking unit(VTU), and a network center with a database of customers who havepurchased a VLU. The network center interfaces with the NationalCriminal Information Center. The entries of that database comprise theVIN number of the customer's vehicle and an identification code assignedto the customer's VLU.

When a LoJack® product customer reports that her vehicle has beenstolen, the VIN number of the vehicle is reported to a law enforcementcenter for entry into a database of stolen vehicles. The network centerincludes software that interfaces with the database of the lawenforcement center to compare the VIN number of the stolen vehicle withthe database of the network center which includes VIN numberscorresponding to VLU identification codes. When there is a match betweena VIN number of a stolen vehicle and a VLU identification code, as wouldbe the case when the stolen vehicle is equipped with a VLU, and when thecenter has acknowledged the vehicle has been stolen, the network centercommunicates with the RTUs of the various communication towers(currently there are 180 nationwide) and each tower transmits a messageto activate the transponder of the particular VLU bearing theidentification code.

The transponder of the VLU in the stolen vehicle is thus activated andbegins transmitting the unique VLU identification code. The VTU of anylaw enforcement vehicles proximate the stolen vehicle receive this VLUtransponder code and, based on signal strength and directionalinformation, the appropriate law enforcement vehicle can take activesteps to recover the stolen vehicle. See, for example, U.S. Pat. Nos.4,177,466; 4,818,988; 4,908,609; 5,704,008; 5,917,423; 6,229,988;6,522,698; and 6,665,613 all incorporated herein by this reference.

Since the VLU operates on a battery, either its own or the vehicle's,there are constraints on the VLU's power consumption. In vehicle poweredconfigurations it must operate in a manner that meets automotiveindustry current drain requirements for devices attached to theautomotive electrical systems. In independent, self-contained batterypowered configurations present goals require operation on a singlebattery for many years, e.g. 10 years. Thus, it is an ever presentchallenge to reduce the magnitude and time of power required.

SUMMARY OF THE INVENTION

In accordance with various aspects of the subject invention in at leastone embodiment the invention presents an improved synchronization systemin a vehicle locating unit in a stolen vehicle recovery system thatkeeps power requirements low enough to meet automotive industryrequirements and/or operate on a single battery for many, many years andenables synchronization with transmitting towers that ensures VLUbattery on-times of as little as 13 ms in 64 seconds.

The subject invention results from the realization, in part, that animproved synchronization system for achieving low power batteryoperation of a vehicle locating unit in a stolen vehicle recovery systemwhich receives periodic transmissions from one or more transmissiontowers can be accomplished using a radio receiver for receiving periodictransmissions; and a microprocessor configured to turn on the receiverfor a limited time to detect an expected message; if an expected messageis not found, turning off the receiver and turning it on again after atime asynchronous with the transmission period; after finding anexpected message, waiting for the period of the transmissions less thelength of an expected message and then looking for a synchronizationsymbol in the expected message and synchronizing subsequent actuation ofthe receiver using that synchronization symbol.

The subject invention, however, in other embodiments, need not achieveall these objectives and the claims hereof should not be limited tostructures or methods capable of achieving these objectives.

This invention features a synchronization system for achieving low powerbattery operation of a vehicle locating unit in a stolen vehiclerecovery system which receives periodic transmissions, including a radioreceiver for receiving periodic transmissions; and a microprocessorconfigured to turn on the receiver for a limited time to detect anexpected message; if an expected message is not found, turning off thereceiver and turning it on again after a time asynchronous with thetransmission period; after finding an expected message, waiting for theperiod of the transmissions less the length of an expected message andthen looking for a synchronization symbol in the expected message andsynchronizing subsequent actuation of the receiver using thatsynchronization symbol.

In preferred embodiments there may be a number of different,geographically arrayed, phased sources of the periodic transmissions andthe microprocessor further may be configured to detect a correspondingsynchronization symbol in a phased transmission from each of the phasedsources, and select one of those phased periodic transmissions from oneof the phased sources to synchronize to. The microprocessor may beconfigured to select for synchronization the phased periodictransmission which has the highest signal strength. The microprocessormay be further configured to recognize when synchronism is lost; checkfor synchronism a predetermined number of times and if synchronism isnot detected waiting again for the period of a transmission less thelength of an expected message and then look for a synchronizationsymbol. The microprocessor may be further configured to recognize whensynchronism is lost; check for synchronism a predetermined number oftimes and if synchronism is not detected waiting again for the period ofa transmission less the length of an expected message and then look fora synchronization symbol. The microprocessor may be further configuredto monitor periodic transmissions from each of the phased sources for anexpected message in at least one of the periodic transmissions. Themicroprocessor may be further configured, if no expected message isfound, to return to turning on the receiver and if an expected messageis found to return to looking for a synchronization symbol in theexpected message.

The invention also features a synchronization method for achieving lowpower battery operation of a vehicle locating unit in a stolen vehiclerecovery system whose radio receiver receives periodic transmissions,including receiving periodic transmissions; turning on a radio receiverfor a limited time to detect an expected message; if an expected messageis not found, turning off the receiver and turning it on again after atime asynchronous with the transmission period; and after finding anexpected message, waiting for the period of the transmissions less thelength of an expected message and then looking for a synchronizationsymbol in the expected message and synchronizing subsequent actuation ofthe receiver using that synchronization symbol.

In preferred embodiments there may be a number of different,geographically arrayed, phased sources of the periodic transmissions andthe looking for a synchronization symbol may include detecting acorresponding synchronization symbol in a phased transmission from eachof the phased sources, and selecting one of those phased periodictransmissions from one of the phased sources to synchronize to. Thephased periodic transmission which has the highest signal strength maybe selected for synchronization. The method may further includerecognizing when synchronism is lost; checking for synchronism apredetermined number of times and if synchronism is not detected waitingagain for the period of a transmission less the length of an expectedmessage and then looking for a synchronization symbol. The method mayfurther include recognizing when synchronism is lost; checking forsynchronism a predetermined number of times and if synchronism is notdetected waiting again for the period of a transmission less the lengthof an expected message and then looking for a synchronization symbol.The method may further include monitoring periodic transmissions fromeach of the phased sources for an expected message in at least one ofthe periodic transmissions. The method may further include, if noexpected message is found, returning to turning on the radio receiverand if an expected message is found returning to looking for asynchronization symbol in the expected message.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Other objects, features and advantages will occur to those skilled inthe art from the following description of a preferred embodiment and theaccompanying drawings, in which:

FIG. 1 is a schematic block diagram of a vehicle locator unit (VLU)including a synchronization system according to this invention;

FIG. 2 is a more detailed illustration of the microprocessor and partsof the VLU of FIG. 1;

FIG. 3 is a state diagram showing the configuration of themicroprocessor of FIGS. 1 and 2 effecting an embodiment of thesynchronization system of this invention;

FIG. 4 is a diagrammatic view of the fields of a burst signaltransmission received by the VLU of FIGS. 1 and 2;

FIGS. 5A and 5B are a state diagram showing in more detail a portion ofthe configuration of the microprocessor of FIGS. 1 and 2 effecting anembodiment of the synchronization system of this invention for attainingsynchronization operating from a “cold start”;

FIG. 6 is a state diagram showing in more detail a portion of a theconfiguration of the microprocessor of FIGS. 1 and 2 effecting anembodiment of the synchronization system of this invention operating tomaintain synchronization;

FIG. 7 is a diagrammatic view of a frame of a burst signal as shown inFIG. 4 illustrating correction of timing as set forth in the statediagram of FIG. 6;

FIG. 8 is a state diagram showing in more detail a portion of a theconfiguration of the microprocessor of FIGS. 1 and 2 effecting anembodiment of the synchronization system of this invention operatingfrom a “warm start”; and

FIG. 9 is a flow chart of a method of synchronization in one embodimentof this invention.

DETAILED DESCRIPTION OF THE INVENTION

Aside from the preferred embodiment or embodiments disclosed below, thisinvention is capable of other embodiments and of being practiced orbeing carried out in various ways. Thus, it is to be understood that theinvention is not limited in its application to the details ofconstruction and the arrangements of components set forth in thefollowing description or illustrated in the drawings. If only oneembodiment is described herein, the claims hereof are not to be limitedto that embodiment. Moreover, the claims hereof are not to be readrestrictively unless there is clear and convincing evidence manifestinga certain exclusion, restriction, or disclaimer.

There is shown in FIG. 1 a vehicle locating unit (VLU) 10 used in astolen vehicle recovery system which receives periodic transmissionsfrom one or more geographically arrayed transmission sources or towers.VLU 10 includes an antenna 12 and antenna tuning unit 14. Antenna 12through antenna tuning unit 14 may receive either a signal fortransmission from RF transmitter 16 or may deliver incoming signals toRF receiver/FM demodulator 18. Antenna switch 20 determines whichcircuit the antenna feed will be connected to. A local RF oscillator 22delivers the signal to transmitter 16 for modulation and to receiver 18for demodulation of the signal. Demodulation may be generally referredto as minimum shift key or MSK and more typically is a SC-MSK or subcarrier modulated MSK. Antenna 12 receives and delivers to receiver 18transmissions from one or more phased transmission sources or towers.For example, if the tower transmission period is 64 seconds and thereare eight such phased sources or towers in a geographical array theneach one will start its 64 second period eight seconds staggered fromthe last. Thus, if there were eight towers numbered one through eightthe first one would start at 64 second period of transmission at timezero, the next tower would start at 64 second period of transmission ateight seconds, after eight more seconds at 16 seconds the third towerwould start its transmission period and so on. The output from RFreceiver/FM demodulator 18 is delivered to decision circuit 24 whichtransforms the sub carrier modulated MSK into a zero crossing signal tobe delivered to microprocessor controller 26. Power is supplied to theVLU through power conditioning circuit 28 which can be connected toeither or both vehicle power at 30 or internal batteries 32. Crystaloscillator 34 establishes a basic clock signal for microprocessor 26 andthe VLU.

Controller or microprocessor 26 includes a timer 36, FIG. 2, responsiveto crystal oscillator 34 and a demodulator 38 which receives the zerocrossing signal on line 40 from decision circuit 24 and demodulates itto ones and zeros. The one and zero signals are submitted to decoder 42which detects short and long flags and short and long preambles whichare a part of the periodic transmission or burst sent by the one or morephased sources and received by the receiver 18 in VLU 10. Bit matchingcircuit 44 operates to review the incoming signal bit for bit andattempt to match it with known stored commands which the receiverexpects. If at any point the bit match fails, the receiver is turned offas it has no further interest in the incoming signal. Also included inmicroprocessor 26 is the synchronization system 50 configuration ofmicroprocessor 26.

At the highest level the operation of synchronization system 50 isdemonstrated by state diagram 60, FIG. 3. Basically it attempts toacquire synchronization from a cold start, 62, if it fails then it loopsfor some period of time, for example, 25 milliseconds trying to acquiresynchronization and if it does not in that time as indicated by loop 64then it shuts down and after a short delay it begins again. This delayshould be a time that it is asynchronous with the transmission periode.g., a multiple or factor of an irrational number. If it has acquiredsynchronization then it moves to state 66 where it maintains thesynchronization. If at any time it loses the synchronization, it movesto state 68 where it attempts, with a so called warm start, to reacquiresynchronization. If it does reacquire synchronization it returns onceagain to state 66; if it doesn't, that is if the warm start fails, thesystem returns to state 62 and once again attempts to acquiresynchronization through a cold start. The acquisition of synchronizationthrough a cold start state 62, the maintenance of synchronization instate 66, and the reacquisition of synchronization with a warm start 68are shown in FIGS. 5A, 5B, 6 and 7, respectively.

The expected message 70, FIG. 4, also known as a burst which istransmitted by the transmission source or tower or by a number of phasedtransmission sources or towers appears diagrammatically in FIG. 4. Tobegin there is noise present 72 followed by a quiet carrier 74.Following this the expected message or burst 70 begins. Typically it hasa duration of 880 ms and occurs every 64 seconds from a particularsource. If there is more than one source, for example, as explained withrespect to FIG. 1, if there are eight towers or transmission sources,each may start their bursts at 8 second intervals. The burst 70 leadingedge 76 is followed by the long preamble 78 and a flag 80. Followingthis is the guaranteed packet message frame or index 82 which has a datasection 83 including a vertical redundancy check 84 and the data 86.After that there are a number of message frames or indexes, for example,ten message frames designated message frame 0-message frame 9. Eachmessage frame is constructed exactly the same and as shown with respectto message frame 0, 88 includes a short preamble 90, a flag 92, and adata section 91 having a vertical redundancy check section 93 and data94.

In one embodiment microprocessor 26 may operate a cold start operationin accordance with the state diagram 100, FIGS. 5A and B. Referring toFIGS. 4 and 5 the receiver is turned on in state 102 for X ms to lookfor a SC-MSK signal. At that point the receiver is turned off.

If it doesn't find an SC-MSK signal it waits Y seconds and tries againand keeps looping 104 until it does find an SC-MSK signal. When thereceiver is turned on and off in state 102, FIG. 5A, the delay periodfrom the time it is turned off until it turned back on again (a periodof Y seconds) should be a period of time which is asynchronous with thetransmission period so that it cannot either immediately or after someperiod of time be turning itself on repeatedly in a periodicnon-productive time slot.

If an SC-MSK signal is found, then the system waits as in state 106 asindicated by loop 108 for the period of one tower cycle, e.g. 64 secondsless or minus a burst time. For example, as indicated with respect toFIG. 4, there may be a burst time of 880 ms so the wait time will be 64seconds minus 880 ms. When the wait is over the system seeks to find anedge in 110. That is, it wishes to encounter preamble 78 and preambleflag 80 of FIG. 4. If it is not found the system loops back as shown at112 to state 102, if it is found the system moves on. Since there may bemore than one transmission source in the area, e.g. a number oftransmission towers, the system enters state 114 where it waits onephase group time and checks for an SC-MSK signal again, repeating thisaction until all 8 phase groups are checked. For example, with a 64second cycle and eight transmission sources in state 114 the system willmove to state 116 and check for the appearance of a MSK every eightseconds and it will record the result. If after checking for MSK signal,it has not checked eight times, it returns along loop 118 to state 114.If it is done checking, that it is, it has done eight checks it moves tostate 120 where it waits for the next one of the phase group which hasbeen found to have an MSK. For example, if there were eight transmissionsource slots and three were found those three starting in state 120would be sought and received again and recorded. The system loops backas indicated at state 122 until all the found MSKs have been reviewedagain. When the receiver messages are received in state 122 they arerecorded especially as to their strength and tower type, and finally instate 126 the best tower or best transmission is selected typically bythe strength of its signal. Mode 1 is the lowest power operating modeand is suitable for networks with very low timing jitter.

For Mode 1 operation the VLU must now receive the first FLAG of thedesired tower's burst, calculate the necessary timing offset to get toits assigned frame, wait the period of time designated by the timingoffset, turn on the receiver to demodulate the short preamble and flagof its assigned message frame, and then receive as much as possible ofthe message frame before bit-matching aborts the operation, and thenshut off the receiver. The VLU will then wait a period equal to theperiod of one tower cycle and repeat the same operation. In this mode ofoperation the VLU will never again (while synchronized) receive thefirst FLAG of the desired tower's burst, it will always turn on justprior to the short preamble and flag of its assigned message frame andattempt to receive the message frame.

For Mode 2 operation all the VLU will always receive the first FLAG ofthe desired tower's burst, calculate the necessary timing offset to getto its assigned frame, wait the period of time designated by the timingoffset, turn on the receiver to demodulate the short preamble and flagof its assigned message frame, and then receive as much as possible ofthe message frame before bit-matching aborts the operation, and thenshut off the receiver. The VLU will then wait a period equal to theperiod of one tower cycle and repeat the same operation. Mode 2operation can be used when operating with tower network equipment thathas much larger jitter.

At that point a synchronization signal is provided 128 (or the system isreturned to state 102 because no good towers were found.) They system isnow synchronized to the transmission signal and also with respect to thespecific message frame 0-9, FIG. 4, that is associated with thisparticular VLU. The synchronization system 50, FIG. 2, will subsequentlyturn on the receiver at precisely the time appropriate to capture theindex or frame 0-9 that is assigned to this particular VLU. For example,if this VLU is assigned to frame 0, 88, FIG. 4, then this system issynchronized to capture the short preamble 90 and then the short flag 92and the data 91.

The synchronization maintenance state diagram 130 is shown in FIG. 6.With the system synchronized 128, FIG. 6, it simply waits for the nextindex or frame wake-up in state 132. When it is time to receive theparticular assigned frame or index it moves to state 134. The receiveris turned on and the message is received.

Message reception is a two step process. The first step is the receptionof the Short Preamble and FLAG fields, and the second step is thereception of the DATA field.

If reception of the Short Preamble and FLAG fields are unsuccessful,Warm Start processing is initiated, the reception of the DATA field isterminated and the receiver is turned off.

If reception of the Short Preamble and FLAG is successful, reception ofthe DATA commences. To minimize power consumption, bit matching is usedto match each bit in the data to be sure that it is a command that isintended for this VLU. If any bit doesn't match it the reception of theremaining part of the DATA field is terminated, the receiver is turnedoff, timing correction is applied, and synchronized reception isscheduled for the next tower transmission.

If the DATA field is a match for one of the messages intended for thisVLU, the system decodes the message, performs the requested function,moves on to state 136 where it corrects for time using the receiverinformation to calculate the next wait and then returns to state 132 towait for the next index wake-up.

The correction of timing in state 136 is depicted with reference to FIG.7. When the receiver is turned on as at 150 a 15 ms delay is introduced152 to give the system a chance to settle. Then a request is made to getthe flag, meaning to find the short flag 92 which appears at thebeginning of each frame. Once the flag is gotten at 154 the time offsetis recorded 156 until the flag is found at 158. That time offset 156 isa fixed time, for example, 13.44 ms. If the flag is not found at exactlythe right time that indicates that the system has drifted and that tomaintain the timing of the VLU coincident with that of the transmissionsource or tower there will have to be an adjustment made.

The warm start 68 previously referred to is shown in more detail in FIG.8. Note the capital letters A, B, C, D, and E on FIGS. 3, 5A, 5B, 6 and7 are intended to show the interaction of the state diagrams 60, 68, 100and 130 in those figures. The synchronization state is checked, a numberof times in state 170, e.g. m times. If the synchronization loss has notoccurred m consecutive times then the system determines that there hastruly not been synchronized loss and returns through connection C FIG.6, to wait for the next index wake-up in state 132. If on the other handthe checking of the synchronization condition in state 170 has exceededm times then the system moves to state 172 where it waits a phase grouptime, e.g. eight seconds. If it hasn't exceeded n iterations then thesystem moves to state 174 where it checks for MSK again. If there is noMSK the system returns to wait for the phase group time. If in state 172the system has exceeded n iterations then it goes to directly to coldstart; if it has found MSK in 174 then it goes to the state 106 in thecold start state diagram 100, FIG. 5A.

Although thus far the embodiment of the invention is in terms of asynchronization system the invention also contemplates a method ofsynchronization for achieving the low power battery operation of avehicle locating unit in a stolen vehicle recovery system which receivesperiodic transmissions as shown in FIG. 9. The synchronization, method200, FIG. 9, begins with turning on the receiver for limited time todetect an expected message from one or more phased transmission sources,202. If no message is found the receiver is turned off 204, and after adelay 206 for a period of time which is asynchronous with thetransmission period, for example a factor or multiple of an irrationalnumber like π, the system returns to step 202 to turn on the receiveronce again. If a message is found 208 then the system waits for thetransmission period less the expected message time 210 and then looksfor a synchronization signal in each phased source transmission.Synchronization is then accomplished for subsequent receiver actuation212 using the synchronization symbol from a selected (typically thehighest signal strength) phased source. If the synchronization is lost acheck for synchronization is made a number m of times 214; if thesynchronism is detected 216 the system returns to normal operation, ifnot it returns to await transmission in step 210.

Although specific features of the invention are shown in some drawingsand not in others, this is for convenience only as each feature may becombined with any or all of the other features in accordance with theinvention. The words “including”, “comprising”, “having”, and “with” asused herein are to be interpreted broadly and comprehensively and arenot limited to any physical interconnection. Moreover, any embodimentsdisclosed in the subject application are not to be taken as the onlypossible embodiments.

In addition, any amendment presented during the prosecution of thepatent application for this patent is not a disclaimer of any claimelement presented in the application as filed: those skilled in the artcannot reasonably be expected to draft a claim that would literallyencompass all possible equivalents, many equivalents will beunforeseeable at the time of the amendment and are beyond a fairinterpretation of what is to be surrendered (if anything), the rationaleunderlying the amendment may bear no more than a tangential relation tomany equivalents, and/or there are many other reasons the applicant cannot be expected to describe certain insubstantial substitutes for anyclaim element amended.

Other embodiments will occur to those skilled in the art and are withinthe following claims.

1. A synchronization system for achieving low power battery operation ofa vehicle locating unit in a stolen vehicle recovery system whichreceives periodic transmissions, comprising: a radio receiver forreceiving periodic transmissions; and a microprocessor configured toturn on said receiver for a limited time to detect an expected message;if an expected message is not found, turning off said receiver andturning it on again after a time asynchronous with said transmissionperiod; after finding an expected message, waiting for the period of thetransmissions less the length of an expected message and then lookingfor a synchronization symbol in the expected message and synchronizingsubsequent actuation of said receiver using that synchronization symbol.2. The synchronization system of claim 1 in which there are a number ofdifferent, geographically arrayed, phased sources of said periodictransmissions and said microprocessor further is configured to detect acorresponding synchronization symbol in a phased transmission from eachof said phased sources, and select one of those phased periodictransmissions from one of said phased sources to synchronize to.
 3. Thesynchronization system of claim 2 in which said microprocessor isconfigured to select for synchronization the phased periodictransmission which has the highest signal strength.
 4. Thesynchronization system of claim 1 in which said microprocessor isfurther configured to recognize when synchronism is lost; check forsynchronism a predetermined number of times and if synchronism is notdetected waiting again for the period of a transmission less the lengthof an expected message and then look for a synchronization symbol. 5.The synchronization system of claim 2 in which said microprocessor isfurther configured to recognize when synchronism is lost; check forsynchronism a predetermined number of times and if synchronism is notdetected waiting again for the period of a transmission less the lengthof an expected message and then look for a synchronization symbol. 6.The synchronization system of claim 5 in which said microprocessor isfurther configured to monitor periodic transmissions from each of saidphased sources for an expected message in at least one of the periodictransmissions.
 7. The synchronization system of claim 6 in which saidmicroprocessor is further configured, if no expected message is found,to return to turning on said receiver and if an expected message isfound to return to looking for a synchronization symbol in the expectedmessage.
 8. A synchronization method for achieving low power batteryoperation of a vehicle locating unit in a stolen vehicle recovery systemwhose radio receiver receives periodic transmissions, comprising:receiving periodic transmissions; turning on a radio receiver for alimited time to detect an expected message; if an expected message isnot found, turning off said receiver and turning it on again after atime asynchronous with said transmission period; and after finding anexpected message, waiting for the period of the transmissions less thelength of an expected message and then looking for a synchronizationsymbol in the expected message and synchronizing subsequent actuation ofsaid receiver using that synchronization symbol.
 9. The synchronizationmethod of claim 8 in which there are a number of different,geographically arrayed, phased sources of said periodic transmissionsand said looking for a synchronization symbol includes detecting acorresponding synchronization symbol in a phased transmission from eachof said phased sources, and selecting one of those phased periodictransmissions from one of said phased sources to synchronize to.
 10. Thesynchronization method of claim 9 in which the phased periodictransmission which has the highest signal strength is selected forsynchronization.
 11. The synchronization method of claim 8 furtherincluding recognizing when synchronism is lost; checking for synchronisma predetermined number of times and if synchronism is not detectedwaiting again for the period of a transmission less the length of anexpected message and then looking for a synchronization symbol.
 12. Thesynchronization method of claim 9 further including recognizing whensynchronism is lost; checking for synchronism a predetermined number oftimes and if synchronism is not detected waiting again for the period ofa transmission less the length of an expected message and then lookingfor a synchronization symbol.
 13. The synchronization method of claim 12further including monitoring periodic transmissions from each of saidphased sources for an expected message in at least one of the periodictransmissions.
 14. The synchronization method of claim 13 furtherincluding, if no expected message is found, returning to turning on saidradio receiver and if an expected message is found returning to lookingfor a synchronization symbol in the expected message.